Overview

Fields and observations can often contain missing values - it can be important to understand the implications of these, and also how to use them to remove unwanted data points. Using a mask of missing values can enable Metview to perform computations on a specific subset of points.

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Computing the mean surface temperature over land

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As an example, we will use a land-sea mask field as the basis of performing a computation on only the land points, excluding all sea points.

Visualise the supplied land_sea_mask.grib icon using the gridvals contour icon. To help illustrate what's going on, we've chosen low-resolution fields - this one is 4x4 degrees. The values are 0 over the sea, 1 over the land and somewhere between 0 and 1 on points which are close to both sea and land. Before we can use this field as a mask, we must do something with those "in-between" points and decide whether they count as land or sea! Let's say that a value of 0.5 or more is land.

Create a new Macro icon and rename it land_points. Type the following code:

lsm = read('land_sea_mask.grib')
lsm = (lsm >= 0.5>